The semiconductor industry's continuing drive toward advanced integrated circuits with increased performance and higher device packing densities has required an accompanying decrease in the minimum feature size of devices. Therefore, each new generation of advanced integrated circuit has required an increase in lithographic resolution. Traditionally, optical lithography has been used to define integrated circuit patterns on semiconductor substrates.
Optical lithography uses a transmission mask in conjunction with a light source to define patterns in a photoresist layer overlying a semiconductor substrate. A conventional transmission mask usually consists of a quartz substrate covered by an opaque composite layer of chrome and chrome oxide that has open slits or apertures defined within it. The chrome oxide that lies on the top surface of the chrome serves as an anti-reflective coating and is essential for high resolution optical lithography because it minimizes the back reflection of light from the mask. A specific configuration or layout of apertures within the composite chrome layer is used to define a specific integrated circuit pattern. The integrated circuit pattern on a transmission mask is transferred to a photoresist coated semiconductor substrate by placing the substrate under the transmission mask, which is then illuminated with a light source. Light passing through the apertures on the transmission mask causes a pattern to be photochemically defined in the photoresist layer. If the photoresist layer has a positive tone, subsequent development of the photoresist layer results in the exposed portion, or the photochemically transformed portion, to be selectively removed with respect to the unexposed portion of the photoresist layer. For a negative tone photoresist, the unexposed portions of the photoresist layer would be selectively removed with respect to the exposed portions of photoresist layer. In either case, the transfer of the integrated circuit pattern from the transmission mask to the photoresist layer, however, is not perfect. This becomes especially true as the distance separating adjacent apertures on the transmission mask decreases.
When light passes through an aperture on the transmission mask, it is diffracted by the composite chrome layer. This results in a partial illumination of the area surrounding the aperture, in addition to the illumination of the area located directly underneath the aperture. Therefore, as the distance between two adjacent apertures is decreased, the light diffracted from each of the apertures constructively interferes. On the semiconductor substrate, this results in the unwanted exposure of photoresist that lies between two exposed regions such that the resulting integrated circuit pattern is either smaller or larger than desired. Consequently, the ability to optically resolve a photoresist pattern with closely spaced features is degraded. Therefore, the continuing reduction of device feature sizes, as required for advanced integrated circuits, is limited.
Phase shifting masks, however, have been proposed for the fabrication of advanced integrated circuits because of their ability to improve the resolution of optical lithography. Phase shifting masks make use of the fact that out of phase light waves destrictively interfere with one another. For example, if the light passing through an aperture is 180 degrees out of phase with light passing through an adjacent aperture, then the diffracted light from both apertures destructively interferes and is canceled in the region between the adjacent apertures. As a result, photoresist patterns with closely spaced features can be resolved.
Although the basic principle of using phase shifting masks to achieve improved optical resolution is relatively straight forward, the actual fabrication of phase shifting masks is more difficult. Many phase shifting masks use a composite layer of chrome and chrome oxide to define phase shifting apertures, similar to transmission masks. Phase shifting masks differ from transmission masks in that the substrate is contoured and non-planar. Thus the substrate is thicker in some areas than in others. The purpose of using a substrate with varying, but controlled, thickness is to achieve the destructive interference associated with phase shifting. Phase shifting is accomplished by decreasing or increasing the path length of light traveling through an aperture with respect to an adjacent aperture. The path length of light traveling through an aperture can be changed by changing the thickness of the substrate appropriately. Typically, thickness differences across a substrate have traditionally been accomplished by forming trenches in the substrate. Thickness differences, however, can also be achieved by forming trenches in a phase shifting layer which overlies the substrate, and which has optical properties which are nearly the same as the substrate. Unfortunately, the existing etches used to form the trenches also etch portions of the chrome oxide anti-reflective coating and thins or roughens the underlying chrome layer. Therefore, chrome-based phase shifting masks suffer from high back reflection and from degraded transmission profiles, since portions of the composite chrome mask are etched. Consequently, the formation of advanced integrated circuits using chrome-based phase shifting masks is limited. Accordingly, a need exists for a process that forms trenched phase shifting apertures, on chrome-based masks, with reduced or no etching of the composite chrome mask.